Update how to share a pack or a game template

[Bouh]

The idea is to make it easy for the end user to share a game template or asset pack, whether paid or free.

TODO:

• [ ] Simplify and explain better the Transaction Management & Invoices management
  • [ ] When can payment be requested for our sales?
  • [ ] Difference EUR / USD
  • [x] The payment methods

Things to fix later:

• [ ] Spine object page is missing
• [ ]